Using the web site, depending on your provider.
On a computer client, depending on the app.
On the iPad, the best you can do is hit the Edit button at the top of the list of emails. You can then tap each email you want to delet to select it, then delele them all with the Delete button.
For now, that's our options, at least with Mail.
There are third party email apps that might have this ability. I'm not familiar with them so can't say. If someone doesn't chime in with a few suggestions you can always go to the App Store and do a little research.